Filter structure and method for filtrating

ABSTRACT

The invention provides a filter structure. A second porous film having a plurality of second holes is disposed on a first porous film having a plurality of first holes. The second holes are smaller than the first holes. The filter structure is dried by an easy and power-saving method such as compression.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application claims priority of Taiwan Patent Application No. 098142841, filed on Dec. 15, 2009, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a filter structure and method for filtrating.

2. Description of the Related Art

A filtration method is a common way to separate solids and liquids. A filter having smaller holes is used to intercept larger solids. The smaller solid and liquid particles pass through the holes. Separating solids and the liquids is thus achieved. However, the conventional filtration method faces a problem where the small holes of the filter are easily blocked by the separated solids in the filtrated solution. Thus the operating lifespan of the filter is very short. Additionally, the small blocked holes need to be washed in order to refresh their ability to function. This requires additional equipment and process and it increases the operating costs. In addition, to increase filtering speed, a high speed-rotating pump is used to produce a compressing or vacuuming process. However, operating the pumping consumes more energy and increases operating costs.

BRIEF SUMMARY OF THE INVENTION

The invention provides a filter structure. A second porous film having a plurality of second holes is disposed on a first porous film having a plurality of first holes. The second holes are smaller than the first holes. The filter structure has excellent venting efficiency and avoids the problem of having blocked holes as a result of filtrated residue. The filter structure can also be dried by an easy and power-saving method such as compression.

The invention also provides a method for filtration. The above-mentioned filter structure is provided. A mixed solution drifts down onto a top surface of the second porous film of the filter structure. Residues larger than the second holes in the mixed solution are intercepted, and a liquid in the mixed solution passes through the second holes and keeps flowing down.

BRIEF DESCRIPTION OF DRAWINGS

The present invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIGS. 1-3 are cross-section views of filter structures according to embodiments of the present invention.

FIGS. 4-5 are cross-sections views of the filter structure during the filtration process.

FIG. 6 shows the apparatus for the filtration experiment.

FIG. 7 shows the experiment result of the Example of the present invention.

FIG. 8 shows the experiment result of the Comparative Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a filter structure for separating solids and liquids. The filter structure has excellent venting efficiency, and avoids the problem of getting blocked holes as a result of filtrated residue. Moreover, the filter structure can be dried by an easy and cost-saving compressing method. References will be made in detail to the present embodiments, and examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the descriptions to refer to the same or like parts. In the drawings, the shape and thickness of one embodiment may be exaggerated for clarity and convenience. The descriptions will be directed in particular to elements forming a part of, or cooperating more directly with, an apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

FIGS. 1-3 are cross-section views of structures of embodiments of the present invention.

Referring to FIGS. 1-2, the filter structure 1A, 1B, for separating the solids and liquids, comprises a porous filter film 11 and porous films capable of absorbing water 12A, 12B. Holes in the porous filter film 11 are smaller than holes in the porous films capable of absorbing water 12A, 12B. It should be understood that in the description, the film capable of absorbing “water” also means a material capable of absorbing “any kind of liquid” without being limited to absorbing “water”, and it is not repeatedly described thereafter for conciseness. The difference between the structures of FIG. 1 and FIG. 2 is that the holes of the porous film capable of absorbing water 12A of FIG. 1 are substantially the same size, and the holes of the porous film capable of absorbing water 12B of FIG. 2 gradually become larger from the top to the bottom of the porous film capable of absorbing water 12B. The porous film capable of absorbing water 12B having holes that gradually becoming larger from the top to the bottom may be formed by stacking a plurality of porous films capable of absorbing water with different-sized holes. The hole size of the porous filter film 11 may be smaller than the solid to be filtrated. For example, the diameter of the hole may be smaller than 0.5 μm for an activated sludge, 10 μm for an activated sludge treated by a coagulation process, and 1 μm for microalgae. The diameters of the holes of the porous film capable of absorbing water 12A, 12B range between diameters of the holes of the porous filter film 11 and 0.457 cm. The porous filter film 11 may comprise a polyvinyl alcohol (PVA), polyethersulfone, cellulose triacetate, polypropylene fiber, polyvinyl chloride, or other suitable materials such as a porous cellulose (for example, regenerated cellulose) or ceramics. The porous films capable of absorbing water 12A, 12B may comprise a polymer such as polyvinyl alcohol (PVA), polyurethane, polyacrylic acid, polyacrylamide, polyethylene or polystyrene, or other suitable foam materials, or may comprise other suitable materials capable of absorbing water, such as non-woven fiber or (synthetic) fiber.

The filter structure 1C shown in FIG. 3 has the porous filter film 11 which is capable of absorbing water 13 and porous supporting film 14. The holes in the porous filter film 11 are smaller than the holes in the porous supporting film 14. The holes in the porous supporting film 14 shown in FIG. 3 are essentially the same size. However, they can also gradually become larger from the top to the bottom of the porous supporting film 14 (not shown). For example, the porous supporting film 14 having holes which gradually become larger from the top to the bottom may be formed by stacking a plurality of porous films capable of absorbing water of different-sized holes. The hole size of the porous filter film 11 may be smaller than the solids to be filtrated. For example, the hole diameter may be smaller than 0.5 μm for an activated sludge, 10 μm for an activated sludge treated by a coagulation process, and 1 μm for a microalgae. The diameters of the holes of the porous supporting film 14 range between the diameters of the holes of the porous filter film 11 and 0.457 cm. The porous filter film 11 may comprise a polymer such as polyvinyl alcohol (PVA), polyethersulfone, cellulose triacetate, polypropylene fiber, polyvinyl chloride, or other suitable materials such as a porous cellulose (for example, regenerated cellulose) or ceramics. The film capable of absorbing water 13 may comprise a polymer such as polyvinyl alcohol (PVA), polyurethane, polyacrylic acid, polyacrylamide, polyethylene or polystyrene, or other suitable foam materials, or may comprise other suitable materials capable of absorbing water, such as non-woven fiber or (synthetic) fiber. The supporting film 14 may comprise the material used for the film capable of absorbing water 13, or other suitable materials having the sufficient physical strength and the holes, such as a metal.

It should be noted that the holes shown in FIGS. 1-3 are the through holes passing through the film, but they also can be voids disorderly distributed in the film (not shown).

The filter structure of the present invention is a multi-layered structure. The holes of the upper filter film (such as the porous filter film 11 shown in FIGS. 1-3) are smaller than the lower film capable of absorbing water (such as the film capable of absorbing water 12A, 12B shown in FIGS. 1-2) or supporting film (such as the supporting film 14 shown in FIGS. 1-3). Therefore, when removing water by compression, a great amount of the liquid flows down, and only a small amount of the liquid flows up and back. To ensure that the liquid flows down smoothly when filtrating, the capillary attraction force difference between the upper and lower films and the water absorption force of the material capable of absorbing water (such as the porous films capable of absorbing water 12A and 12B of FIGS. 1-2 and the supporting film 14 of FIG. 3) can not exceed the gravity force of the filtrated solution itself.

The following description assumes that r represents the diameter of the hole of the upper film (or filter film), and R represents the diameter of the hole of the lower film (or film capable of absorbing water or supporting film) of the filter structure, and γ represents the surface tension, θ represents the contact angle, ρ represents the liquid density, and g represents acceleration of gravity.

In the situation where a filter structure of a single filter film having holes of radius r is used, and during the filtration process, there is still water not filtrated on the surface and only the bottom of the holes is contacted with air, the surface tension force is 2πrγ cos θ, which is an upward drag force. The downward driving force for the filtration process is the gravity force generated by the water in the filter film and the mixed solution not yet filtrated still on the surface of the filter film (the total height of both which is assumed as h₁). In order to make the liquid smoothly flow down, the gravity force of the liquid (πr²ρgh₁) must be bigger than the surface tension force 2πrγ cos θ, in other words, the condition of h₁>(2γ cos θ)/(ρgr) must be conformed. Accordingly, as the diameter (r) increases, the liquid height needed for driving the filtration process decreases. However, the diameter of the hole in fact can not be enlarged infinitely since it is decided depending on the size of the impurities to be filtrated.

In the present invention, the film capable of absorbing water or supporting film (lower film) having the smaller holes is disposed under the filter film (upper film) having the larger holes. The surface tension force associated with the holes of the film capable of absorbing water or supporting film (lower film) is 2πrγ cos θ. Since the diameters of the holes of the lower film are bigger than that of the upper film (R>r), the height (h₂>(2γ cos θ)/(ρgR)) of water to provide a sufficient gravity force to flow downward is small In addition, the effective height of water in the lower film is the total height of the described height h₁ and the thickness of the lower film. Therefore, the driving force for filtrating of the multi-layer filter structure of the present invention is better than the conventional single-layer filter structure.

Moreover, the material having good hydrophility, such as the above-mentioned PVA, can be used for the filter structure of the present invention to increase the attraction force for absorbing water (or any liquid, which will not be repeatedly described here), so as to further increase the driving force for the filtration process. The water in the filter film (upper film) should contact with the film capable of absorbing water (lower film) so that the film capable of absorbing water (lower film) can attract the water by the capillary force. However, if the holes of the lower film are too big to receive the water flowing from the filter structure, the water would directly drop down through the holes of the film capable of absorbing water and not contact with the film. Therefore, such a film capable of absorbing water would not improve filtration efficiency. For example, for a 0.05 ml ball-shaped drop of water, the diameter thereof is about 0.457 cm. Therefore, to receive the water, the lower film needs to have holes with a diameter not exceeding 0.457 cm.

The advantage of the present invention can be illustrated with FIGS. 4-5, which are cross-section views of the filter structure 1A during the filtration process. Referring to FIG. 4, after the mixed solution 30 flows down onto the top surface of the porous filter film 11, the residue 32 in the mixed solution 30 is intercepted by the porous filter film 11 having the holes smaller than the residue 30, and the liquid 31 keeps flowing down through the holes of the porous filter film 11. As the liquid 31 flows to the bottom surfaces of the holes of the porous filter film 11 that are contacted with the porous film capable of absorbing water 12A, the liquid 31 is directly received by the porous film capable of absorbing water 12A due to the capillary attraction of the porous film capable of absorbing water 12A. As the holes of the porous filter film 11 are connected with the holes of the porous film capable of absorbing water 12A, the liquid 31, flowing into the holes of the porous filter film 11, keeps on flowing down into the holes of the porous film capable of absorbing water 12A. Since the holes of the porous film capable of absorbing water 12A are bigger than that of the porous filter film 11, the liquid 31 at the bottom surface of the holes of the porous film capable of absorbing water 12A has a small retarding surface tension force. Therefore, the liquid can keep flowing down smoothly. Accordingly, the filter structure of the present invention has excellent venting efficiency by using the film capable of absorbing water to provide the water-attraction force to increase the flow speed of the filtrated solution. Accordingly, required power is dramatically decreased since the pump that is used in the convention filtration method to increase the flow speed is not needed.

Referring to FIG. 5, after the film capable of absorbing water 12A is full of the liquid 31 (or water, which will not be repeatedly described here), the liquid can be drained out by compressing the film capable of absorbing water 12A. During the compression process, most of the liquid 31 is drained down along the larger holes of the film capable of absorbing water 12A, and a small amount of the liquid 31 flows upward along the smaller holes of the porous filter film 11. The residue 32 on the porous filter film 11 is pushed by the upwardly flowing liquid 31; thereby rinsing the filter structure. It should be noted that the advantage of the present invention is illustrated by the above-mentioned condition, and some conditions, such as when the liquid may be drained out from the sidewall or bottom surface of the porous film capable of absorbing water 12A during the compression process, are not illustrated in detail. After removing the liquid in the porous film capable of absorbing water 12A and removing the compressing force, the compressed porous film capable of absorbing water 12A springs back to its original shape and recovers the ability of absorbing water. The residue 32 accumulated on the porous filter film 11 can be pushed again and again during repeated compression and non-compression. Therefore, the blocking problem described previously can be improved, and the operating lifespan of the filter structure can be increased.

Those skilled in the art should know the advantages of using the filter structure 1B (FIG. 2) and 1C (FIG. 3) while understanding the descriptions associated with FIGS. 4-5. Taking the filter structure 1C of FIG. 3 as one example, after the mixed solution flows down onto the top surface of the porous filter film 11, the residue in the mixed solution is intercepted by the porous filter film 11, and the liquid keeps flowing down through the holes of the porous filter film 11. Since the bottoms of the holes of the porous filter film 11 directly contact with the film capable of absorbing water 13, the liquid in the holes is directly received by the film capable of absorbing water 13.

After the film capable of absorbing water 13 is full of the liquid (or water, which will not be repeatedly described here), the liquid can be drained out by compressing the film capable of absorbing water 13. During the compression process, a small amount of the liquid flows upward along the smaller holes of the porous filter film 11 so as to rinse the residue 32 on the porous filter film 11, and most of the liquid is drained down along the larger holes of the supporting film 14. It should be noted that the advantage of the present invention is illustrated by the above-mentioned condition, and some conditions, such as when the liquid may be drained out from the sidewall of the film capable of absorbing water 13 during the compression process, are not illustrated in detail. After removing the liquid in the film capable of absorbing water 13 and removing the compressing force, the compressed film capable of absorbing water 13 springs back to its original shape and recovers the ability of absorbing water.

The filter structure of the present invention has excellent venting efficiency without an additional force. Thus, power consumption is very low. In addition, the residue blocking process on the filter film can be loose so that it may be removed by repeated compression and non-compression during the filtration process. Therefore, the blocking problem described previously can be improved, and the operating lifespan of the filter structure is increased.

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

Example

The filter structure is a two-layer filter structure, wherein the upper film is a PVA foam filter film having 40 μm diameter holes and the lower film is a PVA foam film capable of absorbing water having 80-120 μm diameter holes.

FIG. 6 shows the apparatus for the filtration experiment. The filter structure 40 is disposed at the bottom opening of the metering barrel, and the sea solution comprising chlorella of concentration of 400 mg/L is put in the metering barrel. The time spent for the solution passing through the filter structure is shown in FIG. 7.

Comparative Example

The filter structure only uses a single PVA foam filter film having 40 μm diameter holes, which are similar to that in the Example. However, no film capable of absorbing water having larger holes under the filter film is provided. The filtration experiment of the single-layer filter structure is performed using an apparatus similar with that in Example. The experiment result is shown in FIG. 8.

Experimental Result

FIG. 7 and FIG. 8 are experiment results of the Example and Comparative Example, respectively. The vertical axis represents the filtering time (min), and the horizontal axis represents the solution volume (ml) passing through the filter structure. From the results shown in FIG. 7 and FIG. 8, it is observed that the filtering speed of the filter structure of the Example is faster than that of the Comparative Example. Accordingly, it is confirmed that the filter structure having the film capable of absorbing water in the present invention has excellent filtration efficiency.

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1. A filter structure, comprising: a first porous film having a plurality of first holes; and a second porous film on the first porous film and having a plurality of second holes, wherein the second holes are smaller than the first holes.
 2. The filter structure as claimed in claim 1, wherein the first porous film capable to absorb water, and the second porous film is capable to filter objects bigger than the second holes.
 3. The filter structure as claimed in claim 1, wherein the first porous film directly contacts the second porous film.
 4. The filter structure as claimed in claim 3, wherein the range for the diameter of the first holes is between diameters of the second holes and 0.457 cm.
 5. The filter structure as claimed in claim 3, wherein the diameters of the second holes are smaller than the diameters of the intercepted solid particles.
 6. The filter structure as claimed in claim 3, wherein the first porous film comprises a polymer, non-woven fiber or synthetic fiber.
 7. The filter structure as claimed in claim 6, wherein the first porous film comprises a polyvinyl alcohol (PVA), polyurethane, polyacrylic acid, polyacrylamide, polyethylene or polystyrene.
 8. The filter structure as claimed in claim 3, wherein the second porous film comprises a polymer or ceramics.
 9. The filter structure as claimed in claim 8, wherein the second porous film comprises a polyvinyl alcohol (PVA), polyethersulfone, cellulose triacetate, polypropylene fiber or polyvinyl chloride.
 10. The filter structure as claimed in claim 1, wherein the first holes of the first porous film gradually become larger from the top to the bottom of the first porous film.
 11. The filter structure as claimed in claim 1, wherein the first porous film having the first holes gradually becoming larger from the top to the bottom of the first porous film is formed by stacking a plurality of porous films capable of absorbing water of different-sized holes.
 12. The filter structure as claimed in claim 1, further comprising a film capable of absorbing water disposed between the first porous film and the second porous film.
 13. The filter structure as claimed in claim 12, wherein the film capable of absorbing water directly contacts the first porous film and the second porous film.
 14. The filter structure as claimed in claim 13, wherein the range for the diameter of the first holes is between diameters of the second holes and 0.457 cm.
 15. The filter structure as claimed in claim 13, wherein the diameters of the second holes are smaller than the diameters of the intercepted solid particles.
 16. The filter structure as claimed in claim 13, wherein the film capable of absorbing water comprises a polymer, non-woven fiber or synthetic fiber.
 17. The filter structure as claimed in claim 16, wherein the film capable of absorbing water comprises a polyvinyl alcohol (PVA), polyurethane, polyacrylic acid, polyacrylamide, polyethylene or polystyrene.
 18. The filter structure as claimed in claim 13, wherein the first porous film comprises a metal, polymer, non-woven fiber or synthetic fiber.
 19. The filter structure as claimed in claim 18, wherein the first porous film comprises a polyvinyl alcohol (PVA), polyurethane, polyacrylic acid, polyacrylamide, polyethylene or polystyrene.
 20. The filter structure as claimed in claim 13, wherein the second porous film comprises a polymer, porous cellulose or ceramics.
 21. The filter structure as claimed in claim 20, wherein the second porous film comprises a polyvinyl alcohol (PVA), polyethersulfone, cellulose triacetate, polypropylene fiber or polyvinyl chloride.
 22. The filter structure as claimed in claim 13, wherein the first porous film has the capability of supporting the second porous film and absorbing water, and the second porous film is capable to filter objects bigger than the second holes.
 23. A method for filtrating, comprising: providing the filter structure as claimed in claim 1; and drifting down a mixed solution onto a top surface of the second porous film of the filter structure to intercept residue larger than the second holes in the mixed solution, and a liquid in the mixed solution passing through the second holes and allow the liquid to continue to flow downward.
 24. The method for filtrating as claimed in claim 23, further comprising: after the first porous film absorbs the liquid, compressing the first porous film by a compressing force to drain out the liquid and rinse the second porous film when the liquid flows back to the second porous film due to the compression to remove the residue blocking the second holes; and removing the compressing force so that the compressed first porous film springs back to an original shape.
 25. The method for filtrating as claimed in claim 23, wherein the filter structure further comprises a film capable of absorbing water disposed between the first porous film and the second porous film, and the method further comprises: after the film capable of absorbing water absorbs the liquid, compressing the film capable of absorbing water by a compressing force to drain out the liquid and rinse the second porous film when the liquid flows back to the second porous film due to the compression to remove the residue blocking the second holes; and removing the compressing force so that the compressed film capable of absorbing water springs back to an original shape. 